Within the food packaging industry it has for a long time been used packages formed from blanks of packaging material, the material being comprised of different layers of paper or board, liquid barriers of for example polymers and gas barriers of for example thin films of aluminium. The blanks are preformed from a material web, which is provided with a pattern of crease lines facilitating forming and folding of packages. The web is cut into pieces, each piece having a size and shape for making one package. After cutting, each piece is folded into a flat tube-formed blank having its longitudinal edges overlapping each other. Next, the longitudinal edges are sealed by any appropriate, conventional sealing technology such as for example heat sealing. The result is a flat tube-formed blank. Forming a blank from a web is well known per se and will not be described in further detail.
In the packaging machine the blank is raised to form a tube usually having a square or rectangular cross section depending on the type of package. Thereafter, one end of the tube can be transversally sealed forming a bottom (or top) of the package and the package is ready to be filled with a product, for example food products like for instance beverages.
Partly formed packages that are open in one end and sealed to form a bottom or top in the other is commonly denoted Ready-To-Fill packages (RTF packages).
To extend the shelf-life of the products being packed it is prior known to sterilize the RTF packages before the filling operation. Depending on how long shelf-life is desired and whether the distribution and storage is made in chilled or ambient temperature, different levels of sterilization can be chosen. One way of sterilizing is to irradiate the inside of the package by electrons emitted from an electron beam emitter. However, irradiation with electrons creates unwanted X-rays. The electrons are first slowed down when passing the electron beam exit window (which will be explained later) and then further slowed down as they collide with amongst others air molecules, bacteria, the package and the walls of the shielding. This decrease of the speed of the electrons gives rise to the emission of X-rays. When such an X-ray hits the shielding, the X-ray enters a certain distance into the material and causes emittance of new X-rays.
So far it has been a problem to obtain acceptable radiation levels outside an irradiation device of reasonable size where RTF packages can pass into and out from in short time.
When using a sterilizing unit such as an electron beam emitter there are also two other issues that usually should be considered. The first consideration is how to safely discharge ozone from the device thereby minimising the risk of ozone leakage to the outside of the device. It is common knowledge that the presence of oxygen molecules (O2) in an electron irradiation device give rise to the formation of ozone during electron irradiation because of radical reactions. Somewhat similar problems arise with sterilization using ultraviolet radiation or chemical sterilization using for instance hydrogen peroxide in gas phase. During use of ultraviolet radiation it is desired to prevent the rays of light from being reflected directly to the outside of the device and when using hydrogen peroxide one wants to isolate the hydrogen peroxide in the sterilizing device and also prevent ozone (O3), created during sterilizing, to leak out of the unit.
The second consideration is how to maintain a desired sterilization level inside the sterilizing device. A device for sterilization of at least partly formed packages is formed with openings for the entrance and exit of packages. Unfortunately, bacteria and other spoilage organisms may enter through the openings and also through interconnections between different portions of the device and the surrounding equipment. If these bacteria and spoilage organisms are left in the device they may recontaminate the packages after they have been sterilized. Moreover, the packages are transported on a conveyor through the machine and the unsterilized packages are removed from the conveyor for sterilization. Afterwards, they are returned to the same conveyor and placed beside still unsterilized packages. Thus, there is also a risk of recontamination of sterilized packages outside the device. It should however be noted that this consideration does not always need to be taken into account. The required level of sterilisation for obtaining a satisfactory shelf-life is different for different types of products and is also, as previously mentioned, depending on whether the distribution and storage is made in chilled or ambient temperature. It has been found that for some products that are not that sensitive, for example juices, and products which are distributed in chilled environment, a satisfactory level of sterilisation, and thereby an acceptable shelf-life, can still be obtained.